Why K9

K9 lets you write load tests in Python, with the script shape, executors, checks, thresholds and metric names you know from k6. The part that sends requests and measures time is written in Rust. Python only describes the work, so most tests never pay for the GIL.

# A K9 script: k9 run script.py
import k9

options = {
    "vus": 50,
    "duration": "1m",
    "thresholds": {"http_req_duration": ["p(95)<300"]},
}

async def default():
    res = await k9.http.get("https://test.k6.io/")
    k9.check(res, {"status is 200": lambda r: r.status == 200})

Why another load tester?

	k6	Locust	Goose	K9
Test language	JavaScript	Python	Rust (compiled)	Python
Engine	Go	Python (gevent)	Rust	Rust
Open-model (arrival-rate) executors	✅	❌ (closed model)	❌	✅
Coordinated-omission handling	Avoided by arrival-rate executors	❌	✅ corrected	✅ both
Load tests inside pytest	❌	❌	❌	✅
License	AGPL-3.0	MIT	Apache-2.0	MIT / Apache-2.0

• From k6: the script shape, the seven executors, checks, the threshold syntax, metric names and exit code 99. Your Grafana dashboards and CI configs carry over.
• From Goose: weighted transactions, coordinated-omission correction, live run control and the manager/worker distributed mode.
• Compared with Locust: Locust is the established Python load tester, and if it serves you well, keep using it. K9 targets cases where the load generator's own overhead or measurement accuracy matters. In Locust the Python process does the HTTP work. In K9 it doesn't. Measured on one machine (docs/benchmarks.md):
• declarative plans send as many requests/s as oha, or more, and 1.4–1.5× k6's, for the least CPU per request;
• one Python process gives 3–4× Locust's single process;
• with --workers, Python scripts give 1.4× Locust's 8 processes at 256 VUs.

How Python is used: three tiers

Tier	What runs in Python	Cost
1. Declarative	Building the plan, before the run	Full Rust speed
2. Hooks	setup, teardown, vu_init, handle_summary	Negligible
3. Iteration functions	default() on every iteration	Async: one event loop per process. Sync: one thread per VU. Scaled across cores with process workers.

k9 inspect tells you which tier a script runs in.